BMP文件

BMP文件剖析

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参考资料

文件分为四大块

Structure	locations
Header	00h - 0Dh
InfoHeader	0Eh - 35h
ColorTable	36h - variable
Pixel Data	variable

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具体分解

Name	Size	Offset	Description
Header	14 bytes		Windows Structure: BITMAPFILEHEADER
Signature	2 bytes	0000h	"BM"
FileSize	4 bytes	0002h	File size in bytes
reserved	4 bytes	0006h	unused (=0)
DataOffset	4 bytes	000Ah	Offset from beginning of file to the beginning of the bitmap data
InfoHeader	40 bytes		Windows Structure: BITMAPINFOHEADER
Size	4 bytes	000Eh	Size of InfoHeader =40
Width	4 bytes	0012h	Horizontal width of bitmap in pixels
Height	4 bytes	0016h	Vertical height of bitmap in pixels
Planes	2 bytes	001Ah	Number of Planes (=1)
Bits Per Pixel	2 bytes	001Ch	Bits per Pixel used to store palette entry information. This also identifies in an indirect way the number of possible colors. Possible values are: 1 = monochrome palette. NumColors = 1 4 = 4bit palletized. NumColors = 16 8 = 8bit palletized. NumColors = 256 16 = 16bit RGB. NumColors = 65536 24 = 24bit RGB. NumColors = 16M
Compression	4 bytes	001Eh	Type of Compression 0 = BI_RGB no compression 1 = BI_RLE8 8bit RLE encoding 2 = BI_RLE4 4bit RLE encoding
ImageSize	4 bytes	0022h	(compressed) Size of Image It is valid to set this =0 if Compression = 0
XpixelsPerM	4 bytes	0026h	horizontal resolution: Pixels/meter
YpixelsPerM	4 bytes	002Ah	vertical resolution: Pixels/meter
Colors Used	4 bytes	002Eh	Number of actually used colors. For a 8-bit / pixel bitmap this will be 100h or 256.
Important Colors	4 bytes	0032h	Number of important colors 0 = all
ColorTable	4 * NumColors bytes	0036h	present only if Info.BitsPerPixel less than 8 colors should be ordered by importance
Red	1 byte		Red intensity
Green	1 byte		Green intensity
Blue	1 byte		Blue intensity
reserved	1 byte		unused (=0)
repeated NumColors times
Pixel Data	InfoHeader.ImageSize bytes		The image data

单个像素信息

Value	Description
1	The bitmap is monochrome, and the palette contains two entries. Each bit in the bitmap array represents a pixel. If the bit is clear, the pixel is displayed with the color of the first entry in the palette; if the bit is set, the pixel has the color of the second entry in the table.
4	The bitmap has a maximum of 16 colors, and the palette contains up to 16 entries. Each pixel in the bitmap is represented by a 4-bit index into the palette. For example, if the first byte in the bitmap is 1Fh, the byte represents two pixels. The first pixel contains the color in the second palette entry, and the second pixel contains the color in the sixteenth palette entry.
8	The bitmap has a maximum of 256 colors, and the palette contains up to 256 entries. In this case, each byte in the array represents a single pixel.
16	The bitmap has a maximum of 2^16 colors. If the Compression field of the bitmap file is set to BI_RGB, the Palette field does not contain any entries. Each word in the bitmap array represents a single pixel. The relative intensities of red, green, and blue are represented with 5 bits for each color component. The value for blue is in the least significant 5 bits, followed by 5 bits each for green and red, respectively. The most significant bit is not used. If the Compression field of the bitmap file is set to BI_BITFIELDS, the Palette field contains three 4 byte color masks that specify the red, green, and blue components, respectively, of each pixel. Each 2 bytes in the bitmap array represents a single pixel.
24	The bitmap has a maximum of 2^24 colors, and the Palette field does not contain any entries. Each 3-byte triplet in the bitmap array represents the relative intensities of blue, green, and red, respectively, for a pixel.